CRUSHING AND GRINDING 209
Then,
Ax = Weight of finished size in the original feed,
By = Weight of finished size in tube-mill feed,
Cy = Weight of finished size in tube-mill discharge, and
Dx = Weight of finished size in the classifier overflow.
In regular operation, with the dry tonnage of the original feed equal to the dry tonnage of the classifier overflow, at any one instant in the cycle the weight of the finished size in the material going to the classifier will equal the weight of finished size in the classifier products. Therefore,
Ax + Cy = Dx -f By
What is required, then, for determining this ratio, is the percentage of undersize from the limiting screen, on the four products as stated above. A few examples from practice will serve as illustrations. In the following work 15 per cent -f 65 mesh, Tyler standard screen, was desired in the classifier overflow.
Example 1. — Oversize in the overflow approximately as desired.
PER CENT
— 65 mesh in the original feed ........................ 38 . 0
—65 mesh in the tube-mill feed ....................... 27 . 8
—65 mesh in the tube-mill discharge ................... 49 . 2
—65 mesh in the classifier overflow .................... 87 . 0
Applying the equation, y = 2.29z.
Example 2. — Oversize in the overflow greater than desired.
PER CENT —65 mesh in the original feed ........................ 29 . 8
—65 mesh in the tube-mill feed ....................... 30 .4
—65 mesh in the tube mill discharge ................... 45 . 8
—65 mesh in the classifier overflow .................... 80 .4
By substituting, as before, y = 3.2Sx.
With a higher allowable amount of oversize in the overflow, the ratio of tube-mill feed to original feed increases, under the stated conditions.
Example 3. — Oversize in the overflow less than desired.
PER CENT
— 65 mesh in the original feed ........................ 29 . 6
— 65 mesh in the tube-mill feed ....................... 35 . 4
— 65 mesh in the tube-mill discharge ................... 81 . 8
—65 mesh in the classifier overflow .................... 94 . 6
Using the equation, y — lAQx.
As the amount of oversize in the overflow is decreased, the ratio of tube-mill feed
to the original feed decreases, under the stated conditions.
It is the general practice in ball-mill grinding to run the feed direct to the mill
instead of to the classifier, as in the cases considered above. With this arrangement,
let
x = Weight of original feed to mill, y = Weight of classifier sands, A = Per cent finished size in original feed, B — Per cent finished size in classifier sands, C = Per cent finished size in mill discharge, and D = Per cent finished size in classifier overflow.